视频慢动作处理是个比较常用的操作，可以在播放的时候处理，这里我们考虑把视频修改为慢动作，使用ffmpeg命令，可以这样

ffmpeg -i test.mp4 -vf "setpts=5*PTS" -an test_slow3.mp4

这里把视频放慢了5倍，生成的文件大小也变大了几倍。

怎么用程序来实现呢，参考一下GPT给出的code，

#include <iostream>
#include <string>
#include <cstdlib>
#include <cstring>
extern "C" {
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libavutil/opt.h>
}// 慢速播放的速度因子
const double SLOWDOWN_FACTOR = 2.0;int main(int argc, char* argv[]) {if (argc < 3) {std::cout << "Usage: " << argv[0] << " input_file output_file" << std::endl;return 1;}const std::string inputFileName = argv[1];const std::string outputFileName = argv[2];av_register_all();AVFormatContext* inputFormatContext = nullptr;if (avformat_open_input(&inputFormatContext, inputFileName.c_str(), nullptr, nullptr) != 0) {std::cerr << "Failed to open input file: " << inputFileName << std::endl;return 1;}if (avformat_find_stream_info(inputFormatContext, nullptr) < 0) {std::cerr << "Failed to retrieve input stream information" << std::endl;avformat_close_input(&inputFormatContext);return 1;}AVFormatContext* outputFormatContext = nullptr;if (avformat_alloc_output_context2(&outputFormatContext, nullptr, nullptr, outputFileName.c_str()) < 0) {std::cerr << "Failed to allocate output format context" << std::endl;avformat_close_input(&inputFormatContext);return 1;}for (unsigned int i = 0; i < inputFormatContext->nb_streams; ++i) {AVStream* inputStream = inputFormatContext->streams[i];AVStream* outputStream = avformat_new_stream(outputFormatContext, nullptr);if (!outputStream) {std::cerr << "Failed to allocate output stream" << std::endl;avformat_close_input(&inputFormatContext);avformat_free_context(outputFormatContext);return 1;}if (avcodec_parameters_copy(outputStream->codecpar, inputStream->codecpar) < 0) {std::cerr << "Failed to copy codec parameters" << std::endl;avformat_close_input(&inputFormatContext);avformat_free_context(outputFormatContext);return 1;}outputStream->time_base = inputStream->time_base;}if (!(outputFormatContext->oformat->flags & AVFMT_NOFILE)) {if (avio_open(&outputFormatContext->pb, outputFileName.c_str(), AVIO_FLAG_WRITE) < 0) {std::cerr << "Failed to open output file: " << outputFileName << std::endl;avformat_close_input(&inputFormatContext);avformat_free_context(outputFormatContext);return 1;}}if (avformat_write_header(outputFormatContext, nullptr) < 0) {std::cerr << "Failed to write output file header" << std::endl;avformat_close_input(&inputFormatContext);avformat_free_context(outputFormatContext);return 1;}AVPacket packet;while (av_read_frame(inputFormatContext, &packet) >= 0) {AVStream* outputStream = outputFormatContext->streams[packet.stream_index];// 慢速播放的时间戳调整packet.pts *= static_cast<int64_t>(SLOWDOWN_FACTOR);packet.dts *= static_cast<int64_t>(SLOWDOWN_FACTOR);packet.duration = static_cast<int>(packet.duration * SLOWDOWN_FACTOR);av_interleaved_write_frame(outputFormatContext, &packet);av_packet_unref(&packet);}av_write_trailer(outputFormatContext);avformat_close_input(&inputFormatContext);avformat_free_context(outputFormatContext);return 0;
}

这个代码执行后并没有实现变慢，参考一下，进行修改后可以实现慢动作处理，如下

int slow() {std::string filename = "test.mp4";     // 输入MP4文件名std::string outputFilename = "test_slow.mp4";  // 输出图片文件名AVFormatContext* ofmt_ctx = nullptr;avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, outputFilename.c_str());const AVOutputFormat* ofmt = ofmt_ctx->oformat;AVFormatContext* formatContext = nullptr;if (avformat_open_input(&formatContext, filename.c_str(), nullptr, nullptr) != 0) {std::cerr << "Error opening input file" << std::endl;return -1;}if (avformat_find_stream_info(formatContext, nullptr) < 0) {std::cerr << "Error finding stream information" << std::endl;avformat_close_input(&formatContext);return -1;}const AVCodec* codec = nullptr;int videoStreamIndex = -1;for (unsigned int i = 0; i < formatContext->nb_streams; i++) {if (formatContext->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {videoStreamIndex = i;codec = avcodec_find_decoder(formatContext->streams[i]->codecpar->codec_id);//AVStream* out_stream = avformat_new_stream(ofmt_ctx, NULL);avcodec_parameters_copy(out_stream->codecpar, formatContext->streams[i]->codecpar);out_stream->codecpar->codec_tag = 0;break;}}avio_open(&ofmt_ctx->pb, outputFilename.c_str(), AVIO_FLAG_WRITE);avformat_write_header(ofmt_ctx, NULL);AVPacket packet;av_init_packet(&packet);// 计算目标时间戳int64_t targetTimestamp = targetSecond * AV_TIME_BASE;// 查找目标时间戳所对应的帧AVFrame* frame = av_frame_alloc();bool foundTargetFrame = false;int count = 0;while (av_read_frame(formatContext, &packet) >= 0) {if (packet.stream_index == videoStreamIndex) {AVStream* inputStream = formatContext->streams[packet.stream_index];AVStream* outputStream = ofmt_ctx->streams[packet.stream_index];// 计算新的时间戳cout << "1, pts=" << packet.pts << endl;packet.pts = av_rescale_q_rnd(packet.pts, inputStream->time_base, outputStream->time_base, AV_ROUND_NEAR_INF);packet.dts = av_rescale_q_rnd(packet.dts, inputStream->time_base, outputStream->time_base, AV_ROUND_NEAR_INF);packet.duration = av_rescale_q(packet.duration, inputStream->time_base, outputStream->time_base);packet.pos = -1;cout << "2, pts=" << packet.pts << endl;// 慢速播放的时间戳调整packet.pts *= 5;packet.dts *= 5;packet.duration *= 5;//          av_log(NULL, AV_LOG_INFO, "...av_write_frame(ofmt_ctx, packet);\n");int ret = av_write_frame(ofmt_ctx, &packet);if (ret < 0) {std::cerr << "Error av_write_frame" << std::endl;}count++;}av_packet_unref(&packet);}av_write_trailer(ofmt_ctx);return 1;
}

通过pts的修改来实现